1. Field of the Invention
The present invention relates to a method for separating zirconium and hafnium tetrachlorides from mixtures thereof as obtained by the direct chlorination of their ores.
2. Description of the Related Art
Zirconium ores such as zirconia contain hafnium in proportions of between 1 and 3% but which can reach 20%. Needles made of zirconium or its alloys which contain, for example, uranium and/or plutonium oxides in nuclear reactors require that hafnium, which has the property of strongly absorbing neutrons, be removed as completely as possible. Most often the hafnium content must be less than 200 ppm, or even 100 ppm. And conversely, in certain parts of reactors (moderators) the hafnium content of the zirconium must be less than 4.5%.
The zirconium and hafnium compounds have very similar physical and chemical properties; the separation methods proposed are numerous. Those which are used industrially include liquid-liquid or aqueous extraction (N. Ozanne, M. L. Lemaire, A. Guy, J. Foos, S. Pellet-Rostaing, F. Chitry, WO, Compagnie Européenne du Zirconium Cezus, Fr., 20010910. 2002, 23 pp.; Oriel, L.; Favre-Reguillon, A.; Pellet-Rostaing, S.; Lemaire, M. Zirconium and hafnium separation, Part 1. Liquid/liquid extraction in hydrochloric acid aqueous solution with Aliquat 336. Separation Science and Technology (2006), 41(9), 1927-1940.). The fractional crystallization of aqueous solutions of fluorozirconate (A highly efficient method for separation of zirconium and hafnium. Ogarev, Vadim; Skotnicki, Antek; Ninham, Barry. (Australia). Aust. Pat. Appl. (2009), 12 pp. CODEN: AUXXCM AU 2008202451 A1 20090108) and of alkaline fluorohafnates or the extractive distillation of the chlorides using an alkaline chloroaluminate at around 360° C. (L. Delons, G. Picard, D. Tigreat, Compagnie Européenne Du Zirconium Cezus, WO 20020412. 2002, 20 pp.; [L. Delons, S. Lagarde, A. Favre Reguillon, S. Pellet Rostaing, M. Lemaire, L. Poriel, Compagnie Européenne du Zirconium-Cezus, Fr.) Fr 2004-7721 2006, 40.). Although selective, these methods suffer from various drawbacks. Thus, the effective extraction of an aqueous phase with methyl ethyl ketone leads to the hydrolysis of the chlorides to oxychlorides that then have to be re-chlorinated in order to allow their reduction with magnesium, for example. The second is costly in terms of labour, investment, reagents and energy. Finally the third, although simpler, is carried out at temperatures so high that it leads to considerable corrosion requiring frequent changes of the trays and internal components of the distillation column.
It is well known that, with the Lewis acids, the most studied of these undoubtedly being aluminium chloride, the aromatic hydrocarbons form arenium complexes the stability of which is all the greater the more basic the aromatic, for example when it contains a higher number of methyl groups. This is also the case with the zirconium and hafnium tetrachlorides. But in these latter cases hexamethyl benzene, taken as an example, reacts with zirconium tetrachloride in order to produce a binuclear molecular complex of formula Zr(η6-Me6C6)Cl2(μ-Cl)3ZrCl3 (F. Musso, E. Solari, C. Floriani, K. Schenk, Organometallics 1997, 16, 4889) whereas hafnium tetrachloride leads to a binuclear ionic complex of formula [Hf(η6-Me6C6)Cl3]+[Hf2Cl9]− (F. Calderazzo, I. Ferri, G. Pampaloni, S. Troyanov, J. Organomet. Chem. 1996, 518, 189; F. Calderazzo, P. Pallavicini, G. Pampaloni, P. F. Zanazzi, J. Chem. Soc., Dalton Trans. 1990, 2743).
At present there is no method that is effective, inexpensive and takes place at temperatures low enough not to result in corrosion.